Commonly used devices for switching laser pulses are optical switches or modulators placed in the beam path within or outside of a laser resonator. Optical modulators include both electro-optical modulators and acousto-optical modulators. An important parameter for selecting the type of modulator is the attainable switching time. Using acousto-optical modulators, the switching time is determined by the speed of sound and the diameter of the laser beam. In some cases these parameters render the attainable switching speed too slow for a number of applications, such as suppressing pre- or post-pulses to a laser pulse that is extracted from an optical cavity. Electro-optical modulators have faster switching speeds and have in recent time been improved, now also allowing the high repetition rates that are attainable with acousto-optical modulators.
As their main element, common electro-optical modulators use a Pockels cell with variable optical properties, and an element sensitive to the polarization direction of the light beam, such as a reflective analyzer with constant optical properties. A Pockels cell normally comprises a birefringent crystal, which is suitably oriented with respect to the propagating light beam and to which a voltage of a few hundred to a few thousand volts is applied for changing the polarization of the incident light beam. Combined with the polarization sensitive element, these elements can be used for switching the light beam propagating through these elements on and/or off, of for changing the path of the light beam by optical element that are sensitive to the polarization of the incident light beam.
Using a suitable high voltage supply that can be switching between two voltage values the Pockels cell can switch the propagating laser beam between two orthogonal states of polarization. The difference of these two voltage values is a function of the parameters of the crystal material that is used and the wavelength of the propagating laser beam to be switched. In some applications both transitions from high to low voltage and vice versa must take place within a few nanoseconds, while some other applications only require one of the transitions to be as fast as a few nanoseconds, whereas the other transition can take place also in a time of microseconds.
Such an electro-optical modulator with a Pockels cell and a switchable high voltage supply can be used to optically switch laser pulses of short duration such as a few nanoseconds (ns), or ultrashort duration such as picoseconds (ps) or femtoseconds (fs), either changing their intensity or their propagation path. Such ultrashort laser pulses can be generated by known principles such as mode locking, Q-switching or gain switching. Mode-locked laser pulses usually have high repetition rates above 1 MHz, typically from 40 MHz to 200 MHz, for solid state lasers, and low pulse energies in the order of nanojoules, typically from 0.1 nJ to 50 nJ. If it is necessary to extract single laser pulses or groups of laser pulses from a continuous ps or fs pulse trains, it is often advantageous to use Pockels cells for this purpose. To do this, the voltage applied to the Pockels cell must be switched completely on or off in the time interval between two pulses, which is typically between 5 ns and 25 ns.
Often ultrashort laser pulses of significantly higher energy than mode-locked laser pulses (e.g., 1,000-fold to 100,000-fold) are necessary for applications such as material processing or nanolinear optical science. In this case, the extracted mode-locked laser pulses are injected into an optical amplifier, passing its amplifying stage the number of times necessary to achieve the desired energy level.
German Patent No. DE 102 51 888 B4 discloses an electrical control for Pockels cells and a laser system employing a Pockels cell with such an electrical control for selecting laser pulses. The electrical control has been modified from the commonly known H-configuration by adding at lest one high voltage switch. This switch can either replace the commonly used recharging resistor or be placed in parallel to it. Such an electrical control can efficiently select individual laser pulses, allowing the precise generation of pulse sequences with defined time intervals for certain applications.
The amplitude of a laser pulse transmitted through an electro-optical modulator is controlled by the amplitude of the voltage signal applied to the Pockels cell. The voltage amplitude can only be varied in time scales in the order of milliseconds. In this way, it is not possible to control the amplitude of consecutive laser pulses, which is desired for certain applications such as material processing.